This application claims priority from Japanese Patent Application No. 2001-400710, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a double-sided printing press that is provided with a sheet-turning-over mechanism for turning sheets upside down for printing on both sides thereof.
2. Related Art
The double-sided printing press of this type includes a sheet-turning-over mechanism for turning sheets upside down. Sheet fed from a sheet-feeding unit to a printing unit on the upstream side of the sheet-turning-over mechanism are printed on, for example, rear sides at the printing unit, and then transferred to the sheet-turning-over mechanism, in which a turn-over gripper grips a tail end of each sheet and then turns it upside down. The sheets each are then printed on a front side at a printing unit on the downstream side, and then transferred to a sheet-discharge unit.
The sheet-turning-over mechanism turns sheets upside down at a sheet-turning-timing corresponding to the length of sheets. That is, since the sheet-turning-over mechanism is designed to allow the turn-over gripper to grip the tail end of an upcoming sheet and turn the sheet upside down, it is necessary to change the timing at which the turn-over gripper grips tail ends of sheets if sheets to be processed have a different length. Therefore, the setting of the length of sheets must be made every time the length of sheets is changed.
Where an operator unintentionally skips an operation to set the length of sheets to be printed or sets a different length, the length of sheets which has been acknowledged by the printing press becomes inconsistent with the length of sheets to be actually printed. As a result, the following problems are caused.
Where the length of sheets to be actually printed is short, the tail end of a sheet passes by the turn-over gripper before it grips the tail end, resulting in failure of sheet transfer to the turn-over gripper. Thus, the sheet which failed to be transferred to the turn-over gripper falls on a lower portion of the sheet-turning-over mechanism.
On the contrary, where the length of an upcoming sheet is long as compared with the length of sheets which is previously set, the tail end of the sheet does not reach a delivering point at which the turn-over gripper timely grips the tail end. This also results in failure of sheet transfer to the turn-over gripper.
Accordingly, it is an object of the present invention to provide a double-sided printing press that is capable of preventing failure of sheet transfer in the sheet-turning-over mechanism.
According to one aspect of the present invention, there is provided a double-sided printing press that includes a sheet-turning-over mechanism for turning sheets upside down at a sheet-turning-timing according to a set length of sheets, which has been previously set for the sheet-turning-over mechanism; and a sheet-length-detection means for detecting the length of sheets, which are fed to the sheet-turning-over mechanism; wherein where the length of sheets, which are fed to the sheet-turning-over mechanism and detected by the sheet-length-detection means, is deferent from the set length of sheets as a result of comparison therebetween, the sheets are stopped from being transferred to the sheet-turning-over mechanism.
Herein, by the length of sheets is meant the length of sheets along sheet transfer direction.
With the printing press having the above arrangement, where the length of sheets to be actually processed is different from the length of sheets set for the sheet-turning-over mechanism, sheet feeding operation is stopped. Accordingly, sheets having a length different from the length of sheets previously set for the sheet-turning-over mechanism are not fed to the sheet-turning-over mechanism. As a result, in the double-sided printing operation, it is possible to prevent a failure of the sheet transfer in the sheet-turning-over mechanism.
Preferably, the sheet-length-detection means is designed to detect the length of sheets based upon the position of a constitutional member of a sheet-feeding section, in which the position of the constitutional member is changed according to the length of sheets, which are fed to the sheet-turning-over mechanism. The position of this constitutional member is adjusted according to the length of sheets every time sheets are newly set in a sheet-feeding section, thereby achieving accurate detection of the length of sheets fed to the sheet-turning-over mechanism.
Preferably, the sheet-length-detection means is designed to detect the length of sheets, which are transferred along a sheet transfer path, based upon the time required for each one of the sheets to pass a predetermined position of the sheet transfer path and a rotational angular displacement of a predetermined cylinder during the time during which each of the sheets to pass the predetermined position.
In this case, for example, the time required for each sheet to pass through a predetermined position of the sheet transfer path can be detected by using a sensor, which is designed for detecting the presence or absence of a sheet positioned therearound. This sensor can be of a simple structure that outputs two values, that is, xe2x80x9conxe2x80x9d representative of the presence of a sheet, and xe2x80x9coffxe2x80x9d representative of the absence of a sheet. This arrangement can also omit the necessity to additionally provide an encoder or the like exclusively used for detecting the rotational angle of a predetermined cylinder, since the printing press is usually provided with the encoder or the like for the purpose of detecting the timing of impression throw-on and throw-off of a cylinder, or any other timing usually employed for a printing operation. An A/D converter circuit can also be omitted. As a result, the sheet-length detection means can be manufactured at low cost because of the arrangement that the length of sheets is detected based upon the sheet passing time and the rotational angle displacement.
Preferably, the sheet-turning-over mechanism includes a storage cylinder and a turn-over cylinder, which are located between two printing units, in which sheets are turned upside down during they are transferred from the storage cylinder to the turn-over cylinder; the rotational phase between the storage cylinder and the turn-over cylinder is changed so as to change the sheet-turning timing; a rotational-phase detection means is provided so as to detect the rotational phase between the storage cylinder and the turn-over cylinder; and the length of sheets, which is determined based upon the rotational phase detected by the rotational-phase detection means, is designated as the set length of sheets which is set for the sheet-turning-over mechanism.
With the above arrangement, the rotational phase corresponding to the sheet-turning-timing is detected, and the length of sheets determined based upon this detected result is designated as the length of sheets set for the sheet-turning-over mechanism. Thus, the length of sheets set for the sheet-turning-over mechanism can be securely and accurately detected.
Preferably, the printing press further includes input means through which the length of sheets is inputted, wherein the length of sheets inputted through the input means is designated as the set length of sheets which is set for the sheet-turning-over mechanism.
A printing press, which automatically switches the sheet-turning-over operation, generally involves inputting the length of sheets and storing the same before starting the sheet-turning-over operation. Therefore, the arrangement that the input means through which the length of sheets is inputted, and this length of sheets inputted through the input means is designated as the length of sheets set for the sheet-turning-over mechanism enables the length of sheets set for the sheet-turning-over mechanism to be found from a value stored in a memory or the like even in the absence of a special means. Thus, a simplified structure can be achieved.